A change in race strategy by the McLaren-Mercedes team was a major contributory factor to Lewis Hamilton's ignominious retirement from Sunday's Canadian grand prix in Montreal, when he ran into the back of the world champion Kimi Raikkonen's Ferrari as he accelerated down the pit lane following a routine refuelling stop.

Martin Whitmarsh, the McLaren chief executive officer, revealed yesterday that Hamilton took on slightly more fuel than his immediate rivals at the fateful pit stop, intending to run a long middle stint of the race before trying to vault back into the lead at the final round of refuelling stops.

He also claimed that Hamilton had been specifically told over the radio that he should watch out for a possible red warning light coming on at the end of the pit lane. At that moment in the race the safety car had been deployed to slow the pace of the field while an abandoned car was moved.

"I've looked into it and we had a little issue when the [pit signalling] board was rotated and the car was dropped down, he nudged [the car] forward a little bit," Whitmarsh told autosport.com. "But I don't think that unduly contributed because the fuel flow was the limiting factor of the stop. The [fuel] nozzle went on and off cleanly.

"We took a view that you can get jumped in a pit lane, but we felt we had the pace and performance that if we were on a reasonably conservative second stint, providing you can stay with the cars in front, you can jump ahead of them at the second stop. In hindsight, it would have been better if there wasn't some cars fighting at the end of the pit lane. Nothing else went wrong."

Whitmarsh stopped short of blaming Hamilton for the incident, admitting that it might have been possible to remind him about the warning lights a few seconds earlier. "Frankly, we gave it to him [but] we could have given it to him earlier," he said. "There was quite a lot going on, obviously, as we were fuelling Lewis's car and that of his team-mate Heikki Kovalainen. When you come in first and come out third I'm sure you're anxious to see if you can jump past those people, and I'm sure that distracted him [Hamilton]. As Nico [Rosberg] proved seconds later, it was easy to do."

Although Whitmarsh picked his words diplomatically, there can be little doubt that McLaren have been left smarting at the 10-place grid penalty imposed on Hamilton for the French grand prix at Magny-Cours on June 22. A similar penalty was imposed on Rosberg, effectively wiping out the two drivers' chances on a track where overtaking is notoriously difficult.

"It's disappointing," said Whitmarsh. "It's a decision the stewards have made so we've got to now make the best we can out of it." Asked how he felt about the decision in the light of the fact that Raikkonen did not incur a penalty when he ran his Ferrari into the back of Adrian Sutil's Force India at Monaco, Whitmarsh replied: "There was a different view taken in Monaco, so we've got to work with what we're given."

With Hamilton now trailing Robert Kubica by four points at the head of the drivers' championship table after BMW Sauber's maiden victory on Sunday, Whitmarsh makes no bones about the fact that he now regards the Switzerland-based team as a title threat. "You've got to," he said. "The reality is they have consistently scored points and done a good job. We've got to recover from a disappointing race here and continue to improve our car.

"We've got to make sure there's no mistakes, we're consistent and we score points. If we can do that, then we'll continue fighting Ferrari but score enough points to overcome BMW."

Hamilton now faces one of the busiest spells of the season, first trying to make the best of his handicap in France before taking in a three-day test at Silverstone the following week in preparation for the British grand prix on July 6. Last year those two races produced key wins for Raikkonen and Ferrari, which began to turn the world championship tide in the Finn's favour. This year Hamilton knows he cannot allow that to happen.